One known projection-type image display device includes an optical system with a digital micromirror device (hereinafter DMD), which is used as an image display element, and a rod integrator. An embodiment of such a known device is described in Japanese Laid-Open Patent Application 2000-206452 and is shown in FIG. 5 of that Laid-Open Patent Application.
Compactness is one of the design criteria for such projection-type image display devices. However, various requirements of such projection-type image display devices limit the compactness of such devices. For example, it is desirable that all the optical elements of the projection-type image display device be on one side of the image display element so that the space on the other side of the image display element is reserved for electrical components. The image display element is mounted on one side of a board and the electrical components are mounted on the other side of the board. It is convenient to have all of the optical elements on one side of the board and all of the electrical components on the other side so that changes in board configuration do not affect the design of the optical elements.
A prior art projection-type image display device includes a projection lens with a shift mechanism for shifting the projection lens in the direction of its optical axis in order to place the projection lens in a desired position so as to provide a bright projected image. A totally reflecting prism is used to illuminate the DMD at an angle of about forty-five degrees and to assist in making the arrangement of the projection lens telecentric.
The properties of a projection-type image display device, such as the specifics of the type of lamp, the F-number of the condenser lens, the size of the rod integrator, and the magnification of the relay optics, are determined for efficient use of the light for forming a bright projected image and to allow freedom in designing the configuration of the projection-type image display device for compactness.
FIG. 3 shows a top view of a known projection-type image display device having a three-dimensional displacement of the light beam. FIG. 4 shows a back view of the projection-type image display device of FIG. 3.
As shown in FIGS. 3 and 4, a light beam (not shown) is reflected by a parabolic reflector 51 and is converged near the entrance end of a rod integrator 55 by a condenser lens 53. The illumination light beam entering the rod integrator 55 is separated into color components in a time-division manner by a color wheel 54 that is positioned near the entrance end of the rod integrator 55. With the light beam intensity homogenized by the rod integrator 55, the illumination light beam is directed into a totally reflecting prism 61, which includes two prisms 60a and 60b, via relay lenses 58a and 58b and mirrors 59a and 59b. The totally reflecting prism 61 totally reflects the incident illumination light beam so as to guide the light to a DMD 62 and then transmits the light beam that is reflected from the DMD to a projection lens 64. Totally reflected on the inner totally reflecting surface of the totally reflecting prism 61, the illumination light beam is incident onto the DMD 62 via a cover glass 63. The light beam that is optically modulated by the DMD 62 in accordance with image signals corresponding to color components in a time-division manner is transmitted through the totally reflecting prism 61 and the projection lens 64 so as to form a magnified image on a screen (not shown).
The prior art projection-type image display device described above is compact. However, recent demands for improved brightness necessitate the use of more powerful light sources and larger reflectors, leading to relatively larger projection-type image display devices and larger optical elements, including larger projection lenses. In order to avoid an increase in overall size, projection-type image display devices must be designed with a configuration different from the prior art configuration described above.
Additionally, recently, projection-type image display devices for individual use have been developed and a so-called home theater market has emerged. For the home theater market, having an appearance that coordinates with other interior designs is often as important as functionality. In this regard, it is desired that the projection-type image display device be thin in a direction that is orthogonal to the optical axis of the projection lens. This direction corresponds to the height of the device and is the vertical direction in FIG. 4.
The present invention relates to a projection-type image display device that uses a DMD as an image display element, is thin in a specified desired direction, and provides efficient use of light so as to obtain a bright projected image. The bright projected image is achieved by selected placement of a circuit board that mounts the DMD, by limiting the incident angles of light to the DMD, and by the design of the relay optics.